Condensate Heat Transfer for Transcritical Carbon Dioxide Refrigeration System

ABSTRACT

A bottle cooler system includes means for using atmospheric water condensate from the evaporator to draw heat from the condenser.

CROSS-REFERENCE TO RELATED APPLICATIONS

Benefit is claimed of U.S. Patent Application 60/663,912, entitled “CONDENSATE HEAT TRANSFER FOR TRANSCRITICAL CARBON DIOXIDE REFRIGERATION SYSTEM” and filed Mar. 18, 2005. Copending application docket 05-258, entitled HIGH SIDE PRESSURE REGULATION FOR TRANSCRITICAL VAPOR COMPRESSION SYSTEM and filed on even date herewith, discloses prior art and inventive cooler systems. The present application discloses possible modifications to such systems. The disclosures of said applications are incorporated by reference herein as if set forth at length.

BACKGROUND OF THE INVENTION

The invention relates to refrigeration. More particularly, the invention relates to beverage coolers.

As a natural and environmentally benign refrigerant, CO₂ (R-744) is attracting significant attention. In most air-conditioning operating ranges, CO₂ systems operate in transcritical mode. An example of a transcritical vapor compression system utilizing CO₂ as working fluid comprises a compressor, a gas cooler, an expansion device, an evaporator and the like (see FIG. 1). The major difference between transcritical and conventional operation is that heat rejection in the gas cooler is in the supercritical region because the critical temperature for CO₂ is 87.8 F. Consequently, pressure is not solely dependent on temperature and this opens additional control and optimization issues for system operation.

FIG. 1 schematically shows transcritical vapor compression system 20 utilizing CO₂ as working fluid. The system comprises a compressor 22, a gas cooler 24, an expansion device 26, and an evaporator 28. The exemplary gas cooler and evaporator may each take the form of a refrigerant-to-air heat exchanger. Airflows across one or both of these heat exchangers may be forced. For example, one or more fans 30 and 32 may drive respective airflows 34 and 36 across the two heat exchangers. A refrigerant flow path 40 includes a suction line extending from an outlet of the evaporator 28 to an inlet 42 of the compressor 22. A discharge line extends from an outlet 44 of the compressor to an inlet of the gas cooler. Additional lines connect the gas cooler outlet to expansion device inlet and expansion device outlet to evaporator inlet.

An electronic expansion valve is usually used as the device 26 to control the high side pressure to optimize the COP of the CO₂ vapor compression system. An electronic expansion valve typically comprises a stepper motor attached to a needle valve to vary the effective valve opening or flow capacity to a large number of possible positions (typically over one hundred). This provides good control of the high side pressure over a large range of operating conditions. The opening of the valve is electronically controlled by a controller 50 to match the actual high side pressure to the desired set point. The controller 50 is coupled to a sensor 52 for measuring the high side pressure.

As the airflow 36 passes over the heat exchanger 28, cooling of the airflow 36 causes the condensation of water out of that airflow. Disposal of that water may need to be addressed. One way involves using the heat rejection heat exchanger to heat the water to induce its evaporation. An example of such a system 60 is shown in FIG. 2.

In the illustrated system 60, components similar to those of the system 20 are shown with like numerals. For illustration, the control and sensor components are hidden. The gas cooler 62 is split into first and second sections 64 and 66. Along the refrigerant flowpath 66, the first section 64 is upstream of the second section 66. The sections 64 and 66 may be along a common air flowpath to receive a common airflow 68 (e.g., driven by a fan 70) or may be on separate air flowpaths (e.g., driven by separate fans). If on a common air flowpath, the first section may be upstream/downstream of the second section.

Water condensed from the airflow 36 is collected by a collection system 80. An exemplary system 80 includes a pan 82 to which the water is delivered. A portion of the first section 64 is positioned to be immersed in a water accumulation in the pan. Heating of the water by the first section 64 encourages evaporation of the water.

SUMMARY OF THE INVENTION

For advantageous performance, however, the condensate may preferably be exposed to a more downstream section of the heat rejection heat exchanger. A bottle cooler system includes means for using atmospheric water condensate from the evaporator to draw heat from the condenser.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a prior art refrigeration system.

FIG. 2 is a schematic view of another prior art refrigeration system.

FIG. 3 is a schematic view of an inventive refrigeration system.

FIG. 4 is a side schematic view of a display case bottle cooler including a refrigeration and air management cassette.

FIG. 5 is a view of a refrigeration and air management cassette.

FIG. 6 is a partial side schematic view of an alternative cassette.

FIG. 7 is a partial side schematic view of an alternative cassette.

FIG. 8 is a partial side schematic view of an alternative cassette.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 3 shows a system 100 having a compressor 22, expansion device 26, and heat absorption heat exchanger (evaporator) 28. These may be similar to corresponding components of the systems of FIGS. 1 and 2. For illustration, the control and sensor components are hidden. The gas cooler 102 is split into first and second sections 104 and 106. Along the refrigerant flowpath 66, the first section 104 is upstream of the second section 106. The sections 104 and 106 may be along a common air flowpath to receive a common airflow 108 (e.g., driven by a fan 110) or may be on separate air flowpaths (e.g., driven by separate fans). In the exemplary system, the first section 104 is upstream of the second section 106 with the fan 110 intervening.

Water condensed from the airflow 36 is collected by a collection system 112. An exemplary system 112 includes a pan 122 to which the water is delivered. A portion of the second section 106 is positioned to be immersed in a water accumulation in the pan 122. Heating of the water by the second section 64 may encourage evaporation of the water. Contrasted with the system of FIG. 2, the section of the gas cooler which gives up heat to the condensate is relatively downstream along the refrigerant flow path (e.g., in the cooler half or quarter of the temperature drop prior to the expansion device). This is intended to reduce the refrigerant temperature as much as possible by exposing the coldest refrigerant to the condensate. For a transcritical CO₂ refrigeration system, to maintain peak efficiencies it is critical to minimize the temperature at the exit of the high-side (gas cooler) heat exchanger.

It is even more critical to minimize this exit temperature for a CO₂ bottle cooler refrigeration system. Manufacture costs are of particular concern. The result is that low cost/relatively lower efficiency heat exchangers (including but not limiting to wire-on-tube heat exchanger, plate-on-tube heat exchanger, finless heat exchanger etc.) are particularly useful for to control bottle cooler manufacture costs.

Thus, a particular area for implementation of the condensate heat exchange is in bottle coolers, including those which may be positioned outdoors or must have the capability to be outdoors (presenting large variations in ambient temperature). FIG. 4 shows an exemplary cooler 200 having a removable cassette 202 containing the refrigerant and air handling systems. The exemplary cassette 202 is mounted in a compartment of a base 204 of a housing. The housing has an interior volume 206 between left and right side walls, a rear wall/duct 216, a top wall/duct 218, a front door 220, and the base compartment. The interior contains a vertical array of shelves 222 holding beverage containers 224.

The exemplary cassette 202 draws the air flow 108 through a front grille in the base 224 and discharges the air flow 108 from a rear of the base. The cassette may be extractable through the base front by removing or opening the grille. The exemplary cassette drives the air flow 36 on a recirculating flow path through the interior 206 via the rear duct 210 and top duct 218.

FIG. 5 shows further details of an exemplary cassette 202. The heat exchanger 28 is positioned in a well 240 defined by an insulated wall 242. The heat exchanger 28 is shown positioned mostly in an upper rear quadrant of the cassette and oriented to pass the air flow 36 generally rearwardly, with an upturn after exiting the heat exchanger so as to discharge from a rear portion o the cassette upper end. A drain 250 may extend through a bottom of the wall 242 to pass water condensed from the flow 36 to the drain pan 122. A water accumulation 254 is shown in the pan 122. The pan 122 is along an air duct 256 passing the flow 108 downstream of the heat exchanger first section 104. The heat exchanger second section 106 is positioned to be at least partially immersed in the accumulation 254. Exposure of the accumulation 254 to the immersed second section 106 and to the heated air in the flow 108 may encourage evaporation.

In an exemplary, coil routing of the second section 106, the second section is divided into a first portion normally above the accumulation and in the airflow 108 and a second portion normally immersed. The refrigerant flow path may pass generally downstream along the air flow 108 through the first portion and then pass into the second portion before proceeding to the expansion device.

The FIG. 5 arrangement is consistent with a basic reengineering of a baseline cassette having a single heat rejection heat exchanger located where the first section 104 is and nothing where the second section 106 is. It is also consistent with a reengineering of a split system where the hotter section is in that latter position. However, the illustrated configuration has the disadvantage that the cooler section is downstream of the hotter section along the air flow path. Accordingly, it may be desirable to reverse the air flow to become back-to-front. A portion of this back-to-front air flow could be directed to flow over the cooler door window to avoid window fogging.

An alternative implementation might eliminate the physical separateness of the first section 104. One example would be to only have a single heat rejecting heat exchanger unit positioned as represented by the second section 106 in FIG. 5. The immersed portion of that exchanger unit could serve the role of the second section 106 while the exposed portion could serve the role of the first section 104 (see FIG. 6 below). Another simple variation could involve heat exchanger positioning so that water dripping from the drain flows over a leading portion of the heat exchanger (i.e., at the upstream end of the warm air flow).

Various implementations may further maximize heat transfer via a counterflow exchange of condensate water and the refrigerant. This counterflow may be the exclusive method of heat exchange between the condensate and the refrigerant, or may supplement pan immersion or another mechanism. FIG. 6 shows such a system, wherein the drain 250 having an outlet 260. A length 262 of the refrigerant line extends upward to the outlet. The length 262 is positioned to guide/wick droplets of water from the outlet 260 downwardly along the length 262 to the drain pan. With refrigerant flowing upward through the length 324, the refrigerant and water are in counterflow heat exchange. A more upstream (along the refrigerant flow path) length 264 (or portion of the heat rejection heat exchanger) may be immersed in the water 254 in the pan. a yet more upstream portion 270 may be in the air flow

In another example of a supplementary situation, a relatively small downstream section of the gas cooler may run through/in the drain pan 122. A smaller yet more downstream portion may run up into the to evaporator drain in a counterflow heat exchange (both along its length and/or merely a two step counterflow in combination with the portion in the pan). In the FIG. 7 example, the drain 250 is replaced by a more convoluted drain 300. The drain 300 has an upwardly directed U-portion 302 defining a water trap containing a water slug 304. The drain 300 and slug 304 may prevent air leakage between the hot and cold air flows and might be used independently in place of the simpler drain 250. The slug is continuously replenished by condensate flowing into the drain 300 and continuously discharges condensate down toward the pan 122. A portion 306 of the refrigerant line extends from a remainder of the second section 106 and through the drain 300. The expansion device (not shown) may be positioned between the downstream end of the line portion 306 and the evaporator 28. Thus refrigerant flowing through the line portion 306 is in counterflow heat exchange with the condensate flowing through the drain 300. Although shown piercing the drain 300, the line portion 306 may enter the drain outlet 308 and/or exit the drain inlet 310 and more closely follow the path of the drain.

FIG. 8 shows an alternate drain 320 having an outlet 322. A length 324 of the refrigerant line extends upward to the outlet. The length 324 is positioned to guide/wick droplets of water from the outlet 322 downwardly along the length 324 to the drain pan. With refrigerant flowing upward through the length 324, the refrigerant and water are in counterflow heat exchange. A more upstream (along the refrigerant flow path) portion of the heat rejection heat exchanger may be immersed in the water in the pan.

In other implementations, the condensate could be delivered to air flow (e.g., 108) just prior to its passing over the last portion of the heat rejecting heat exchanger (i.e., the gas cooler which would be a condenser if conditions were appropriate) so that the heat transfer is enhanced and hence the refrigerant temperature is reduced. This may be particularly effective in dry climates where evaporative cooling of the air flow is particularly relevant.

This condensate to air delivery could be done in several ways. A wick could be placed upstream of the relevant section of the heat exchanger along the air flow. A spray device could be similarly positioned to introduce the spray of condensate to the air flow. Such a spray could also or alternatively directly contact the relevant heat exchanger portion to cool via evaporative or conventional cooling. Similarly, a wick could contact the heat exchanger to transport the water and provide conventional and/or evaporative cooling.

Thus, it is seen that for transcritical bottle cooler applications, the water being condensed on evaporator surfaces is useful for refrigerant cooling to maintain efficiency. This approach especially provides additional efficiency for low cost, fouling resistant, heat exchangers like wire-on-tube, plate-on-tube, finless heat exchangers, and the like. This may enable performance comparable to high efficiency finned-tube conventional heat exchangers currently being used for bottle cooler applications. The protective coating typically present on low cost heat exchangers (wire-on-tube, plate-on-tube, etc.) may provide effective resistance to corrosion from the condensate to which the heat exchanger is exposed.

One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented as a remanufacturing of an existing system or reengineering of an existing system configuration, details of the existing configuration may influence details of the implementation. Exemplary baseline systems could be transcritical CO2 systems or could have other operational domains and/or other refrigerants. Accordingly, other embodiments are within the scope of the following claims. 

1. A cooler system comprising: a compressor (22) for driving a refrigerant along a flow path in at least a first mode of system operation; a first heat exchanger (102) along the flow path downstream of the compressor in the first mode so as to act as a condenser; a second heat exchanger (28) along the flow path upstream of the compressor in the first mode so as to act as an evaporator to cool contents of an interior volume of the system; and means for using atmospheric water condensate from the second heat exchanger to draw heat from a downstream portion of the first heat exchanger.
 2. The system of claim 1 wherein the means comprises: immersion of said downstream portion (106) of the first heat exchanger (102) in a drain pan (122) of the second heat exchanger (28).
 3. The system of claim 1 wherein the means comprises at least one of: a wick conveying the water condensate to said downstream portion; a wick conveying the water condensate to an airflow flowing over said downstream portion; at least a first subportion of the downstream portion of the first heat exchanger extending upward to receive a flow of the water condendsate and guide said flow to a drain pan; and at least a first subportion of the downstream portion of the first heat exchanger extending upward to receive a flow of the water condensate and guide said flow to a drain pan with a second subportion in the pan.
 4. The system of claim 1 wherein the means comprises: a sprayer for spraying the water condensate onto the first heat exchanger.
 5. The system of claim 1 wherein the means comprises: a counterflow heat exchange between refrigerant and a flow of the water condensate.
 6. The system of claim 1 being a self-contained externally electrically powered beverage cooler positioned outdoors.
 7. The system of claim 1 wherein: the refrigerant comprises, in major mass part, CO₂; and the first and second heat exchangers are refrigerant-air heat exchangers.
 8. The system of claim 1 wherein: the refrigerant consists essentially of CO₂; and the first and second heat exchangers are refrigerant-air heat exchangers each having an associated fan, an air flow across the first heat exchanger being an external to external flow and an airflow across the second heat exchanger being a recirculating internal flow.
 9. The system of claim 1 in combination with said contents which include: a plurality of beverage containers in a 0.3-4.0 liter size range.
 10. The system of claim 9 being selected from the group consisting of: a cash-operated vending machine; a transparent door front, closed back, display case; and a top access cooler chest.
 11. The system of claim 1 being a transcritical system. 